Well known methods for producing microwave and millimeter wavelength radiation involve the use of electron tubes which rely on various forms of velocity modulation of an electron beam, followed by a drift section to achieve density bunching. After bunching, the electron kinetic energy is converted into microwave or millimeter waves. In klystrons, this is achieved with two-cavity or multi-cavity arrangements. However, the bandwidth is limited in this case. To achieve wider bandwidths traveling wave velocity modulation is more appropriate. In traveling wave tube amplifiers (TWTA's) electrons interact with the longitudinal electric component of a slow electromagnetic wave. The phase velocity of the electromagnetic wave is slowed down to match the electron velocity, and thereby continuous and cumulative interaction between the electron beam and the wave can take place producing microwaves or millimeter waves over bandwidths as wide as one octave or more. These devices are referred to as slow wave structures. The simplest slow wave structure used in many wide bandwidth TWTA's is the helix waveguide. Bandwidth over one octave is common. Other slow wave structures are also well known.
Electron beam density modulation in a TWTA is characterized by the bunching parameter .chi. which is defined by: ##EQU1## where E.sub.o is the magnitude of the modulating electric field, V.sub.o is the electron beam voltage, k is the longitudinal wavenumber (2.pi./wavelength) of the traveling wave, and l is the length of the TWT. Maximum electron density bunching at the end of the TWT structure occurs for .chi.=1. At .chi.=1.84 the bunching is optimum in the sense that the modulated electron beam contains maximum modulation at the fundamental frequency. It is an easy matter to adjust both E.sub.O and l to achieve the desired bunching characteristics.
Traveling wave-tube-amplifier (TWTA) technology is a relatively mature technology which was developed several decades ago. One of the limits of the TWTA is its output power which falls off rapidly as the frequency is increased into the millimeter wave range. Average power of the current state-of-the-an millimeter TWTA ranges from the hundreds of watts to 1 kW.
The applicant has patented a high power fast switch (U.S. Pat. No. 4,993,033) which discloses a diamond switch which is switched on by illumination with a pulsed electron beam. Embodiments disclosed in that patent were expected to produce 10-100 kW pulses at a 10-100 MHz repetition rate. Experiments have demonstrated the operation of such a diamond switch. The switched electric pulse follows the waveform of the illuminating electron beam pulses.
Traditional sources of microwave and millimeter wave radiation have been based on solid state devices or vacuum electron devices. Although solid state devices have shown great promise as the source for various applications, they have generally failed to deliver power greater than several watts and efficiencies greater than a few percent. Vacuum electron devices on the other hand generate greater power with higher efficiency, but they tend to be very bulky. What is needed is a light-weight compact high power (i.e. several kilowatts) microwave source which can operate within the wavelength range from about 3 millimeters to about 300 millimeters or frequencies from 1 GHz to about 100 GHz.